The present disclosure relates to a separator and a battery and, more particularly, to a microporous separator and a battery using such a separator.
Owing to the development of a recent portable electronic apparatus technique, electronic apparatuses such as cellular phones, notebook-sized computers, and the like are appreciated as a basic technique which supports an advanced information society. Studies and development regarding a technique for realizing advanced functions of those electronic apparatuses are vigorously being progressed. Electric power consumption of those electronic apparatuses is also increasing in proportion. On the contrary, it is requested that those electronic apparatuses can be driven for a long time and it is inevitably requested to realize high energy density of a secondary battery serving as a driving power source.
From viewpoints of an occupied volume, a mass, and the like of a battery which is built in the electronic apparatus, the larger energy density of the battery, the better. At present, lithium ion secondary batteries are built in most of the apparatuses because they have excellent energy density.
Generally, in the lithium ion secondary battery, lithium cobalt acid is used for a cathode, a carbon material is used for an anode, and an operating voltage is set to a value within a range from 2.5 to 4.2 V. In a single battery, a terminal voltage can be raised to 4.2V owing to an excellent electrochemical stability of a non-aqueous electrolyte material, a separator, and the like.
On the other hand, in the lithium ion secondary battery in the related art which operates at the maximum voltage of 4.2V, a cathode active material such as lithium cobalt acid or the like which is used for the cathode merely uses a capacitance of about 60% of its theoretical capacitance and a residual capacitance can be utilized in principle by further raising a charge voltage. Actually, it has been known that the high energy density can be realized by setting the voltage upon charging to 4.25V or more (for example, refer to a pamphlet of International Publication No. WO03/019713).
In such a lithium ion secondary battery, although the battery has been designed so as to assure sufficient safety under the ordinary using conditions, in order to cope with the realization of the recent high capacitance, higher safety is requested.
In a lithium ion secondary battery having a winded laminate structure, in order to realize higher safety, there has been proposed a technique to use a structure in which cathode/anode collector exposed portions are made to face cathode/anode outermost peripheral portions by a length of one circumference or more (for example, refer to JP-A-1996 (Heisei 8)-153542. According to such a lithium ion secondary battery, even if it is erroneously used, a sudden temperature increase of the battery can be suppressed by an inner short-circuit due to a contact of metals whose electric resistances are sufficiently small. The lithium ion secondary battery having such a structure has already been put into practical use and it has been confirmed that the excellent effect is obtained.